1. Field of the Invention
The present invention generally relates to power transistors, and more specifically to a hybrid bipolar/field-effect power transistor fabricated in the group III-V material system.
2. Description of the Related Art
The insulated gate bipolar transistor (IGBT) is a hybrid power transistor which offers the high input impedance and low input current of a field-effect transistor, together with the high current capability of a bipolar transistor. A general description of the IGBT is presented in a textbook entitled "POWER TRANSISTORS: DEVICE DESIGN AND APPLICATIONS", edited by B. Jayant Baliga et al, IEEE Press, 1984, pp. 354-363.
A conventional IGBT 10 is illustrated in FIG. 1, and is fabricated in silicon. The IGBT 10 includes a bipolar PNP type transistor section 12 including a P+ emitter layer 14, an N- base or drift layer 16 and a plurality of laterally spaced P+ collector layers 18. Portions of the base layer 16 extend upwardly into the lateral gaps between the collector layers 18 to constitute vertical injection regions 20.
An anode terminal 22 is ohmically connected to the emitter layer 14 via a metal contact layer 24. N+ regions 26 are formed in the opposite end portions of the collector layers 18, and provide ohmic contact between the layers 18 and cathode terminals 28 via metal contacts 30.
The IGBT 10 further includes a field-effect transistor (FET) section 31 including a plurality of insulated gate FET structures 32. Each structure 32 includes a gate insulator layer 33 which is formed of silicon dioxide or silicon nitride and extends over the underlying injection region 20 between adjacent N+ regions 26. A gate metal layer 34 is formed over each insulator layer 33 to provide a metal-oxide-semiconductor (MOS) gate structure.
The metal layers 34 are connected to gate terminals 36. For high power operation, the cathode terminals 28 are connected together and the gate terminals 36 are connected together. The cathode terminals 28 are grounded, and a positive voltage is applied to the anode terminal 22.
With zero voltage applied to the gate terminals 36, lateral channels 38 defined in the upper portions of the collector layers 18 under the gate insulator layers 33 between the adjacent lateral edges of the N+ regions 26 and the collector layers 18 are depleted of electrons, and no electron current flows through the channels 38.
Application of a sufficiently high positive voltage to the gate terminals 36 enhances (attracts electrons into) the channels 38 and the upper portions of the injection regions 20 which constitute extensions of the channels 38. This creates a lateral conduction path from the N+ regions 26 into the injection regions 20 through the channels 38.
Electrons are injected from the cathode terminals 28 through the lateral conduction path from which they are vertically injected into the injection regions 20. The electrons drift downwardly from the regions 20 through the base layer 16 and emitter layer 14 toward and are collected at the anode terminal 22. The electron flow through the base layer 16 increases the conductivity thereof, and causes holes to be back injected from the anode 22 and emitter layer 14 into the base layer 16, from which they flow through the collector layers 18 to the cathode terminals 28.
The hole current from the anode terminal 22 through the emitter layer 14, base layer 16 and collector layers 18 to the cathode terminals 28 constitutes the main power current of the IGBT 10. The base layer 16 is conductivity modulated by the electron current which is vertically injected into the regions 20 from the channel regions 38 of the FET section 31. The larger the electron injection current, the higher the conductivity of the base layer 16 and the higher the hole current of the bipolar transistor section 12.
The FET section 31 has an insulated gate or MOS structure, as this enables the highest power operation of the various field-effect transistor configurations using silicon technology. However, the conventional silicon IGBT has relatively low electron mobility, which produces a high forward voltage drop across the device and accompanying power loss.
In addition, the low bandgap of silicon produces a relatively low breakdown voltage. Another problem inherent in silicon IGBTs is a latch-up phenomenon caused by parasitic NPN transistor or PNPN thyristor action. These drawbacks prevent the silicon IGBT from being used in applications such as power switches in which these limitations are unacceptable.
Transistor devices fabricated in group III-V material systems, including gallium arsenide (GaAs) and indium phosphide (InP), have higher electron mobility, higher breakdown voltage, higher operating speed and lower losses. However, the IGBT design cannot be practically implemented in a group III-V material system as it requires the gate insulator layers 33 which are difficult or impossible to fabricate on group III-V materials.